Alkyne-assisted nanostructure growth

ABSTRACT

The present invention relates to the formation and processing of nanostructures including nanotubes. Some embodiments provide processes for nanostructure growth using relatively mild conditions (e.g., low temperatures). In some cases, methods of the invention may improve the efficiency (e.g., catalyst efficiency) of nanostructure formation and may reduce the production of undesired byproducts during nanostructure formation, including volatile organic compounds and/or polycylic aromatic hydrocarbons. Such methods can both reduce the costs associated with nanostructure formation, as well as reduce the harmful effects of nanostructure fabrication on environmental and public health and safety.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/817,818, filed Jun. 17, 2010, which claims priority under 35 U.S.C.§119(e) to co-pending U.S. Provisional Application Ser. No. 61/187,704,filed Jun. 17, 2009, the contents of which are incorporated herein byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with the support under the following governmentcontract: CMMI0800213 awarded by the National Science Foundation. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to nanostructures, and related methods.

BACKGROUND OF THE INVENTION

The industrial and laboratory-scale production of carbon nanotubes(CNTs) has been increasing for the last decade, with current productionvolumes at 1,300 tons year⁻¹ globally (ton=10⁶ g) (e.g., with a doublingrate of once every two years and assuming that the CNT mass: US $ ratiois unchanged over the next ten years). Many known methods forlarge-volume CNT production, including catalytic chemical vapordeposition (CVD), are plagued by inefficiency where no more than about3% of the introduced carbon feedstock is converted to CNT. In somecases, the unused feedstock is recycled for subsequent nanotube growth,but in many other cases, the effluent and its associated by-products arevented to the atmosphere. These untreated materials could amount to anannual release of 41,000 tons (41×10⁹ g) of carbonaceous material, andthis may expand to 1,300,000 tons year⁻¹ (1.3×10¹² g year⁻¹) within thenext decade if production accelerates as predicted.

Additionally, for many methods, heating the feedstock gas at hightemperatures is necessary for rapid CNT growth in order to generatecritical CNT precursor molecules. Recent studies have demonstrated thatthe effluent from an ethene-based CVD growth, i.e., by thermal treatmentof common CNT feedstock gasses (C₂H₄/H₂), contained several compoundsthat pose threats to the quality of the air, water and soil. Theseinclude toxics (e.g., benzene, 1,3-butadiene, and aromatichydrocarbons), greenhouse gases (e.g., methane), and compounds thatcontribute to smog formation and exacerbate respiratory illness.

SUMMARY OF THE INVENTION

The present invention provides methods for forming carbonnanostructures. In some embodiments, the method comprises contacting areactant vapor comprising a nanostructure precursor material with acatalyst material to cause formation of nanostructures, wherein thenanostructure precursor material comprises at least one hydrocarbon andthe reactant vapor is maintained at a temperature of less than 400° C.prior to contacting the catalyst material.

In some embodiments, the method comprises contacting a reactant vaporcomprising a nanostructure precursor material with a catalyst materialto cause formation of nanostructures, wherein the reactant vapor issubstantially free of an oxygen-containing species or anitrogen-containing species, and the nanostructures are formed with acatalyst efficiency of about 1×10² grams of nanostructure/grams ofcatalyst material or greater.

In some embodiments, the method comprises introducing a reactant vaporcomprising a nanostructure precursor material into a reaction chamber,the reactant chamber comprising a catalyst material; contacting thereactant vapor with the catalyst material to cause formation ofnanostructures and a product vapor comprising at least onecarbon-containing byproduct; and allowing the product vapor to exit thereaction chamber, wherein the product vapor comprises the at least onecarbon-containing byproduct in an amount less than 10% of the totalvolume of product vapor that exits the reaction chamber during formationof the nanostructure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a graph illustrating the correlation between the amount ofthermally generated methane and increases in VA-MWCNT growth rate.

FIG. 1B shows a graph illustrating the correlation between the amount ofthermally generated benzene and increases in VA-MWCNT growth rate.

FIG. 1C shows a graph illustrating the correlation between the amount ofthermally generated methyl acetylene and increases in VA-MWCNT growthrate.

FIG. 1D shows a graph of CNT growth rate as a function of the partialpressure for some thermally generated compounds, including methane,vinyl acetylene, benzene, and methyl acetylene.

FIG. 2A shows a graph illustrating the effects of chemical structure onVA-MWCNT growth with test gases delivered, in combination with a mixtureof unheated ethene/hydrogen/helium, to a heated metal catalyst at equalpartial pressures (9.8×10-3 atm), except for vinyl acetylene, which wasmore dilute (3.0×10-3 atm).

FIG. 2B shows a graph illustrating the effects of chemical structure onVA-MWCNT growth with test gases delivered, in combination with a mixtureof unheated ethene/hydrogen/helium, to a heated metal catalyst at lowerpartial pressures (3.3×10-4 atm).

FIG. 2C shows a graph illustrating the effects of chemical structure onVA-MWCNT growth with test gases were delivered, in combination with amixture of unheated ethene/hydrogen/helium, to a heated metal catalystat equal masses (5.5±0.4 ug C sccm-1).

FIG. 3 shows a plot of the partial pressures of various reactants and byproducts, including volatile organic compounds (VOCs), during ananostructure formation process.

FIG. 4 shows a plot of the partial pressures of various reactants and byproducts, including polycyclic aromatic hydrocarbons (PAHs), during ananostructure formation process.

FIG. 5A shows a plot of nanostructure growth rate as a function ofethene (or ethylene) partial pressure during acetylene-assisted CNTgrowth.

FIG. 5B shows a plot of catalyst lifetime as a function of ethylenepartial pressure during acetylene-assisted CNT growth.

FIG. 5C shows a plot of nanostructure growth rate as a function ofhydrogen partial pressure during acetylene-assisted CNT growth.

FIG. 5D shows a plot of catalyst lifetime as a function of hydrogenpartial pressure during acetylene-assisted CNT growth.

FIG. 6A shows a schematic diagram of a reactor system, according to oneembodiment.

FIG. 6B shows a schematic diagram of an atmospheric-pressure, cold-wallCVD reactor with decoupled thermal control over feedstock and catalyst.

FIG. 7A shows a proposed mechanism for carbon nanotube (CNT) growth.

FIG. 7B shows possible propagation steps wherein elongation of the CNToccurs by sequential alkyne or alkene addition.

FIG. 8 shows a proposed mechanism for CNT termination.

FIG. 9 shows a graph of carbon conversion yield for various reactionconditions in the CVD reactor.

FIG. 10 shows a graph of CNT product purity as a function of G/D ratiofor various reaction conditions in the CVD reactor.

Other aspects, embodiments and features of the invention will becomeapparent from the following detailed description when considered inconjunction with the accompanying drawings. The accompanying figures areschematic and are not intended to be drawn to scale. For purposes ofclarity, not every component is labeled in every figure, nor is everycomponent of each embodiment of the invention shown where illustrationis not necessary to allow those of ordinary skill in the art tounderstand the invention. All patent applications and patentsincorporated herein by reference are incorporated by reference in theirentirety. In case of conflict, the present specification, includingdefinitions, will control.

DETAILED DESCRIPTION

The present invention generally relates to the formation and processingof nanostructures, including nanotubes, and related systems and methods.Some embodiments described herein involve simplified processes fornanostructure growth and/or utilize relatively mild conditions. Suchmethods can both reduce the costs associated with nanostructureformation, as well as reduce the harmful effects of nanostructurefabrication on environmental and public health and safety.

In some cases, methods of the invention advantageously provide theability to reduce the production of undesired byproducts duringnanostructure formation, including volatile organic compounds and/orpolycylic aromatic hydrocarbons, thereby reducing the total amount ofcarbonaceous material being vented to the atmosphere. Some embodimentsdescribed herein also provide the ability to form nanostructures withoutpre-treatment (e.g., thermal pre-treatment) of the nanostructureprecursor material. For example, the method may involve incorporation ofone or more additives (e.g., alkyne additives) within the nanostructureprecursor material that may facilitate nanostructure formation withoutneed for thermal pre-treatment of the precursor material. Additionally,methods described herein may improve the efficiency (e.g., catalystefficiency) of nanostructure formation, which can also aid in reducingharmful emissions produced during nanostructure formation. Such methodscan reduce manufacturer costs (e.g., feedstock costs), as well as reduceadditional losses due to production bans, environmental remediationefforts, and the like.

In some embodiments, methods for forming nanostructures are provided. Insome cases, the method may involve a chemical vapor deposition process.For example, the method may involve contacting a reactant vapor with acatalyst material, and allowing the reactant vapor to undergo a chemicalreaction with the catalyst material to produce a desired product. Insome embodiments, gaseous precursor materials, selected for theirability to be converted to a particular desired product, may beintroduced directly to a catalyst material in order to form the desiredproduct in high yield and to reduce the formation of potentially harmfuland unintended byproducts. For example, a reactant vapor comprising ananostructure precursor material may contact a catalyst material (e.g.,a metal or metal oxide catalyst material, a non-metal catalystmaterial), causing formation of nanostructures, such as nanotubes. Insome embodiments, the reactant vapor may comprise various components,including hydrocarbons (e.g., ethylene), hydrogen, helium, alkyneadditives, and other components, as described more fully below. In somecases, the reactant vapor may be maintained under relatively mildconditions (e.g., room temperature) prior to contacting the catalystmaterial, i.e., the reactant vapor is not thermally pre-treated.

Methods of the invention may generally comprise formation or growth ofnanostructures on the surface of a catalyst material. In someembodiments, the catalyst material may be arranged on or in the surfaceof a substrate. In some embodiments, the catalyst may be in powder form.Upon exposure of the catalyst material to a reactant vapor under a setof conditions selected to facilitate nanostructure growth,nanostructures may grow from catalyst material. Without wishing to bebound by theory, the mechanism of nanostructure formation may involve(1) nucleation, wherein a nanostructure precursor material contacts thecatalyst material to form a nanostructure cap or ananostructure/catalyst species; (2) elongation, where additionalnanostructure precursor material, such as various carbon units, can addto the growing nanostructure; and (3) termination, where a chemicalevent (e.g., reductive elimination of hydrogen or water), mechanicalstress, catalyst encapsulation, and/or catalyst deactivation may haltnanostructure growth.

Without wishing to be bound by theory, FIGS. 7-8 show an illustrativeembodiment for the mechanism of nanostructure formation. For example,FIG. 7A shows a proposed CNT growth mechanism. During the anneal phaseof CNT production, the catalyst was reduced and subject to morphologicalchanges that yield metal nanoparticles. Previous studies have described(1) coupling reactions between alkynes and alkenes that proceed viametallocycles and (2) alkane insertion to a growing C-chain. In somecases, excess hydrogen atoms are liberated without increasing the C—Cbond order. FIG. 7B, continued from FIG. 7A, shows possible propagationsteps wherein elongation of the CNT occurs by sequential alkyne oralkene addition. An unstable transition state is depicted, where excesshydrogen atoms are liberated by reaction with radicals (R·) andresultant electrons add to the CNT lattice and ultimately reduce themetal catalyst.

FIG. 8 shows a proposed mechanism for CNT termination. Previous studieshave indicated that water can potentially cleave catalyst-CNT bonds. Insome cases, excess gas-phase hydrogen could also terminate growth.

The methods described herein may be useful in the formation of a widerange of nanostructures, including carbon nanotubes. In someembodiments, the nanostructures are single-walled carbon nanotubes ormulti-walled carbon nanotubes. In some embodiments, the method mayproduce a set of substantially aligned nanostructures formed on thesurface of the catalyst material. As used herein, a “set ofsubstantially aligned nanostructures” refers to nanostructures which areoriented such that their long axes are substantially non-planar orsubstantially non-parallel with respect to the surface of the catalystmaterial and/or substrate. In some cases, the nanostructures arearranged on or in a surface of the catalyst material, such that the longaxes of the nanostructures are oriented in a substantially non-parallel(e.g., substantially perpendicular) direction with respect to thesurface of catalyst material and/or substrate (e.g., a nanostructure“forest”). In some embodiments, the nanostructures are verticallyaligned multi-walled carbon nanotubes. In other embodiments, thenanostructures may be arranged on or in a surface of the catalystmaterial, such that the long axes of the nanostructures aresubstantially parallel to the surface. For example, the nanostructuresmay be formed or grown with their long axis along the surface of asubstrate.

Using methods described herein, nanostructures having high purity may beformed. For example, nanostructures may be formed with reduced amountsof carbon-containing byproducts, catalyst impurities, and othernon-nanostructure materials in the final product. In some embodiments,the product may comprise at least 30%, at least 40%, at least 50%, atleast 60%, at least 70%, at least 80%, at least 90%, or, in someembodiments, at least 95% nanostructures, as determined by thermalgravimetric analysis. In one set of embodiments, the product maycomprise about 80% to about 95% (e.g., 84-94%) nanostructures, asdetermined by thermal gravimetric analysis.

As described herein, some embodiments provide methods where the reactantvapor is maintained under relatively mild conditions (e.g., withoutplasma or thermal treatment), prior to contacting the catalyst material.In previous methods, the reactor vapor is typically thermally treated athigh temperatures (e.g., >400° C.) prior to contact with a catalystmaterial, and/or is exposed to the catalyst material while maintained athigh temperatures, in order to successfully produce nanostructures.However, as a result, many undesired byproducts are produced. Anadvantageous feature of the present invention is that such thermalpre-treatment of the reactant vapor (e.g., nanostructure precursormaterial) may be eliminated, substantially reducing or preventing theformation of unwanted by-products (e.g., toxics, greenhouse gases, andsmog-forming compounds), as well as reducing energetic demands andimproving overall control over the synthesis.

In some cases, methods are provided for the catalytic formation ofnanostructures using reactant vapors maintained at relatively lowertemperatures (e.g., below 400° C.). For example, the reactant vapor mayinclude a nanostructure precursor material, wherein at least some of thenanostructure precursor material may undergo the chemical reaction uponexposure of the catalyst material to the reactant vapor. In someembodiments, the reactant vapor may be maintained at temperatures lessthan 400° C., prior to contacting the catalyst material. In someembodiments, the reactant vapor is maintained at a temperature of lessthan 400° C., less than 300° C., less than 200° C., less than 100° C.,less than 75° C., less than 50° C., less than 30° C., or, in some cases,less than 25° C., prior to contacting the catalyst material. In somecases, the reactant vapor comprises a nanostructure precursor materialand is maintained at a temperature of less than 400° C., such that atleast 10%, 25%, 50%, 75%, or greater, of the nanostructure precursormaterial may undergo the chemical reaction upon exposure to the catalystmaterial.

In some embodiments, the method may advantageously allow for theproduction of nanostructures with reduced formation of undesiredbyproducts, including various carbon-containing byproducts. As usedherein, the term “byproduct” refers to an undesired or unintendedspecies that may be formed during a reaction catalyzed by the catalyticmaterial. For example, in the context of the invention, a byproductgenerally refers to any product of a chemical or thermal reaction (e.g.,a reaction that is catalyzed by the catalyst material), that is not ananostructure. A byproduct, however, does not refer to species thatundergo essentially no net chemical alteration or transformation uponexposure to a catalyst material (e.g., are “unreacted”), but that maylater be recovered as unreacted starting material. For example, areactant gas comprising ethylene may be introduced into a reactionchamber comprising a catalyst material to produce a nanostructureproduct, and any unreacted ethylene that exits the reaction chamber isnot considered a byproduct. In some cases, an undesired byproduct is aspecies that can adversely affect certain properties of the desiredreaction product, i.e., the nanostructures, or may otherwise be harmfulto public health or the environment. For example, previous methods ofnanostructure formation often produced large quantities of undesiredcarbon-containing byproducts, including volatile organic compounds andpolycyclic aromatic hydrocarbons, both of which can pose various healthand environmental dangers. In some cases, such carbon-containing byproducts were formed at high temperatures (e.g., >400° C.).

In some embodiments, methods described herein may provide the ability toform nanostructures with significant reduction in the formation ofbyproducts. For example, the method may involve contacting, in areaction chamber, a reactant vapor with a catalyst material, resultingformation of a product vapor, wherein the product vapor includes atleast one, undesired carbon-containing byproduct in an amount less than10% of the total volume of product vapor that exits the reaction chamberduring formation of the nanostructure. In some cases, the product vaporincludes at least one carbon-containing byproduct in an amount less than5%, less than 2.5%, or less than 1% of the total volume of product vaporthat exits the reaction chamber during formation of the nanostructure.

In some cases, the product vapor comprises one or more volatile organiccompounds in an amount less than 10%, less than 5%, less than 3%, lessthan 2.6%, less than 2%, less than 1.5%, less than 1%, or less than 0.9%of the total volume of product vapor that exits the reaction chamberduring formation of the nanostructure. In embodiments where ethylene isa nanostructure precursor material, the volatile organic compound is notethylene and the amount of volatile organic compound present in theproduct vapor is calculated to exclude ethylene. As used herein, theterm “volatile organic compound” is given its ordinary meaning in theart and refers to an organic chemical species having a sufficiently highvapor pressure to evaporate at room temperature, thereby entering theatmosphere. In some cases, the term “volatile organic compound” excludesspecies which, in a particular process, are present in the reactantvapor as nanostructure precursor materials, such as ethylene. Examplesof volatile organic compounds include hydrocarbons such as alkanes,alkenes, aromatic compounds, and the like, such as methane, ethane,propane, propene, 1,2-butadiene, 1,3-butadiene, 1,3-butadiyne, pentane,pentene, cyclopentadiene, hexene, or benzene. In some embodiments, thevolatile organic compound is methane, 1,3-butadiene, or benzene.

In some embodiments, where methane is not a nanostructure precursormaterial and is substantially not present in the reactant vapor, theproduct vapor comprises methane in an amount less than 10%, less than5%, less than 2.5%, less than 1%, less than 0.5%, less than 0.4%, lessthan 0.3%, less than 0.2%, less than 0.1%, less than 0.05%, or, in somecases, less than 0.01%, of the total volume of product vapor that exitsthe reaction chamber during formation of the nanostructure.

In some embodiments, where 1,3-butadiene is not a nanostructureprecursor material and is substantially not present in the reactantvapor, the product fluid comprises 1,3-butadiene in an amount less thanless than 10%, less than 5%, less than 2.5%, less than 1%, less than0.5%, less than 0.4%, less than 0.3%, less than 0.2%, less than 0.1%,or, in some cases, less than 0.05%, of the total volume of product vaporthat exits the reaction chamber during formation of the nanostructure.

In some embodiments, where benzene is not a nanostructure precursormaterial and is substantially not present in the reactant vapor, theproduct fluid comprises benzene in an amount less than less than 10%,less than 5%, less than 2.5%, less than 1%, less than 0.5%, less than0.3%, less than 0.1%, less than 0.05%, less than 0.01%, less than0.005%, less than 0.001%, less than 0.0005%, or, in some cases, lessthan 0.0001%, of the total volume of product vapor that exits thereaction chamber during formation of the nanostructure.

In some embodiments, the amount of volatile organic compound generatedduring nanostructure formation may be reduced by a factor of 10, 20, 30,40, 50, 60, of greater, relative to previous methods, including methodswhere the reactant vapor is heated to high temperatures (e.g., 400° C.or greater).

In some cases, the product vapor comprises one or more polycyclicaromatic hydrocarbons formed as an undesired byproduct. As used herein,the term “polycyclic aromatic hydrocarbon” is given its ordinary meaningin the art and refers to carbon species comprising a fused network ofaromatic rings. The polycyclic aromatic hydrocarbon may be substantiallyplanar or substantially non-planar, or may comprise a planar ornon-planar portion. The term “fused network” might not include, forexample, a biphenyl group, wherein two phenyl rings are joined by asingle bond and are not fused. Generally, the polycyclic aromatichydrocarbons may include four-, five-, six-, or seven-membered rings.However, it should be understood that rings of other sizes may beincluded. Examples of polycyclic aromatic hydrocarbons includenaphthalene, acenaphthalene, fluorene, phenanthrene, anthracene,fluoranthene, pyrene, chrysene, coronene, triphenylene, naphthacene,phenanthrelene, picene, fluorene, perylene, or benzopyrene. In someembodiments, the use of methods described herein may reduce the amountof polycyclic aromatic hydrocarbon generated during nanostructureformation by 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or more,relative to previous methods, including methods where the reactant vaporis heated to high temperatures (e.g., 400° C. or greater).

In some cases, the product vapor includes carbon-containing byproductswhich contain two carbon atoms (e.g., ethane), three carbon atoms (e.g.,propane), or more.

In some embodiments, the methods described herein may allow forformation of nanostructures with high catalyst efficiency. For example,the nanostructures may be formed with a catalyst efficiency of about1×10² grams of nanostructure/grams of catalyst material or greater. Insome cases, the nanostructures may be formed with a catalyst efficiencyof about 2×10², 3×10², 4×10², 5×10², 6×10², 7×10², 8×10², 9×10², 1×10³,or greater. In some embodiments, the nanostructures may be formed with acatalyst efficiency of about 5×10² to 1.1×10³. The catalyst efficiencymay also be improved, relative to previous methods, by use of a reactantvapor that is substantially free of an oxygen-containing species or anitrogen-containing species, as described herein.

The reactant vapor and/or catalyst material may be exposed to a set ofconditions suitable for facilitating the production or growth ofnanostructures. As used herein, exposure to a “set of conditions” maycomprise, for example, exposure to a particular temperature, pH,solvent, chemical reagent, type of atmosphere (e.g., nitrogen, argon,helium, oxygen, etc.), electromagnetic radiation, other source ofexternal energy, or the like, for a period of time. In some cases, theset of conditions may be selected to facilitate nucleation, growth,stabilization, removal, and/or other processing of nanostructures. Insome cases, the set of conditions may be selected to facilitatereactivation, removal, and/or replacement of the catalyst material. Insome cases, the set of conditions may be selected to maintain thecatalytic activity of the catalyst material. Some embodiments mayinvolve a set of conditions comprising exposure to a source of externalenergy, including electromagnetic radiation, electrical energy, soundenergy, thermal energy, or chemical energy.

As described herein, in some embodiments, the reactant vapor may bemaintained at a temperature below about 400° C. prior to exposure to thecatalyst material. In some embodiments, the reactant vapor may bemaintained at a temperature less than about 300° C., 200° C., 100° C.,75° C., 50° C., 30° C., or, in some cases, less than about 25° C., priorto contacting the catalyst material. For example, this may be achievedby use of an apparatus capable of separately controlling/maintaining thetemperatures of the reactant vapor and the catalyst substrate. (FIG. 6B)That is, the apparatus may be capable of locally heating the reactantvapor and/or catalyst material. In some cases, it may desirable tomaintain the reactant vapor and the catalyst material at similartemperatures. In some cases, it may desirable to maintain the reactantvapor and the catalyst material at different temperatures. In someembodiments, it may be desirable to minimize the heating of the reactantvapor during nanostructure formation. However, it should be understoodthat the methods and systems described herein may be useful forreactions conducted at temperatures greater than 400° C. In someembodiments, a reaction employing reactant vapors and/or catalystmaterials described herein may be performed at greater than 600° C.,700° C., 800° C., greater than 900° C., or greater.

Some embodiments involve maintaining the catalyst material at a certaintemperature during nanotube formation. In some cases, the catalystmaterial may be maintained at a temperature of at least 50° C., at least100° C., at least 200° C., at least 300° C., at least 400° C., at least500° C., at least 600° C., at least 700° C., at least 800° C., at least900° C., or, in some cases, at least 1000° C., or greater. Thetemperature of the catalyst material may be maintained by arranging thecatalyst material in combination with a temperature-controlledsubstrate, such as a resistively heated silicon platform.

In some cases, the method may involve pre-heating the reactant vapor,prior to exposure to the catalyst material. The reactant vapor may thenbe cooled to and maintained at a temperature below 400° C. In somecases, the reactant vapor may be pre-heated to a temperature of at leastabout 60° C., about 80° C., about 100° C., about 200° C., about 300° C.,about 400° C., about 500° C., about 600° C., about 700° C., about 900°C., about 1000° C., about 1100° C., about 1200° C., about 1300° C.,about 1400° C., about 1500° C., or greater. In one set of embodiments,the reactant vapor may be pre-heated to a temperature between about 700°C. and about 1200° C. The pre-heated reactant vapor may then be cooledto a temperature below about 400° C. (e.g., room temperature), andintroduced into a reaction chamber comprising the catalyst material. Itshould be understood that, in some cases, it may be desirable for themethod to not include a pre-heating step.

Some embodiments of the invention may comprise the use of an additivethat may enhance formation of the nanostructures. For example,incorporation of an alkyne additive in the reactor vapor may increasethe growth rate of the nanostructures. In some embodiments, the use ofan alkyne additive within the reactant vapor may eliminate the need forthermal pre-treatment of the reactant vapor. As used herein, the term“alkyne” is given its ordinary meaning in the art and refers to achemical species containing at least one carbon-carbon triple bond(e.g., “—C≡C—”). The alkyne may comprise one, two, three, or foursubstituents, any of which may be optionally substituted. In some cases.the alkyne may comprise less than 10, less than 7, or, in some cases,less than 5 (e.g., 4) carbon atoms. The alkyne additive may compriseadditional groups, including alkene groups (e.g., carbon-carbon doublebonds), i.e., an en-yne group. Examples of alkyne species includeacetylene (or “ethyne”), methyl acetylene (or “propyne”), vinylacetylene (or “but-1-en-3-yne”), 1,3-butadiyne, or the like. In somecases, the use of an alkyne additive in the reactant vapor may producenanostructures (e.g., nanotubes) at an accelerated growth rate, relativeto previous methods (e.g., lacking the alkyne additive). In someembodiments, the incorporation of an alkyne additive may increase thenanostructure growth rate by 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%,90%, or greater.

In an illustrative embodiment, the method may involve introducing areactant vapor comprising a nanostructure precursor material and analkyne and maintained at low temperature (e.g., at less than about 400°C., or, in some cases, at room temperature) into a reaction chambercomprising a catalyst material, wherein the reactant vapor has not beensubjected to pre-heating. The reactant vapor may then be contacted witha catalyst material to cause formation of nanostructures, as well as aproduct vapor comprising at least one carbon-containing byproduct. Thecarbon-containing by product may be formed in an amount that is at leasttwo, three, four, five, six, seven, eight, nine, or ten times lower thanin a product vapor that is formed from an essentially identical reactantvapor lacking the alkyne. In some embodiments, a reactant vaporcomprising the nanostructure precursor material and the alkyne may notbe subjected to a pre-treatment step (e.g., a pre-heating step) and maybe contacted with a catalyst material to form nanostructures and aproduct vapor containing a significantly reduced amount ofcarbon-containing byproduct, relative to a product vapor formed from areactant vapor that has been pre-treated (e.g., pre-heated) prior tocontacting the catalyst and that lacks the alkyne. In some cases, anon-pre-heated reactant vapor comprising an alkyne may producecarbon-containing byproduct molecules in amounts that are ten times lessthan a pre-heated reactant vapor lacking an alkyne.

As noted above, the reactant gas may comprise various components. Insome embodiments, the reactant gas comprises a nanostructure precursormaterial. As used herein, a “nanostructure precursor material” refers toany material or mixture of materials that may be reacted to form ananostructure under the appropriate set of conditions, such as exposureto a catalyst material. In some cases, the nanostructure precursormaterial includes a gas or mixture of gases. For example, thenanostructure precursor material may comprise a hydrocarbon (e.g., C₂H₄and CH₄, etc.), one or more fluids (e.g., gases such as H₂, O₂, helium,argon, nitrogen, etc.), or other chemical species that may facilitateformation of nanostructures (e.g., alkynes).

In some embodiments, the reactant vapor comprises a hydrocarbon,hydrogen, and optionally an alkyne additive. For example, the reactantvapor may include 35% or less hydrocarbon, wherein the hydrocarbon isnot an alkyne, 70% or less hydrogen, and optionally 10% or less, 1% orless, or 0.1% or less alkyne additive, by volume. In some cases, thereactant vapor comprises 35% or less ethylene, 70% or less hydrogen, and0.1% or less alkyne, by volume. Where the sum of the relative amounts ofhydrocarbon, hydrogen, and alkyne additive, if present, do not equal100%, additional components may be included in the reactant vapor, suchas helium or argon, or mixtures thereof, to bring the total to 100%.

In some embodiments, the reactant vapor comprises about 16% to about 35%ethylene, by volume. In some embodiments, the reactant vapor comprisesabout 16% to about 70% hydrogen, by volume. In some embodiments, thereactant vapor further comprises helium. For example, the reactant vapormay comprise 20% ethylene, 51% hydrogen, and 29% helium, by volume. Insome embodiments, the reactant vapor further comprises argon. In someembodiments, the reactant vapor further comprises a mixture of heliumand argon.

In another set of embodiments, the reactant vapor may include ethylene,hydrogen, and an alkyne.

In some cases, the reactant gas is substantially free of anoxygen-containing species, such as an alcohol, an ether, a ketone (e.g.,acetone), an ester, an amide, or the like, or a nitrogen-containingspecies, such as an amine. As used herein, “substantially free of anoxygen-containing species or a nitrogen-containing species” means thatthe reactant gas includes less than 1%, less than 0.5%, or less than0.1%, or less than 0.01%, by volume, of an oxygen-containing species ornitrogen-containing species. In one set of embodiments, the reactantvapor is substantially free of acetone. That is, acetone is not presentas a co-reactant in, for example, nanostructure formation. In one set ofembodiments, the reactant vapor is substantially free of ethanol. Insome cases, the oxygen-containing species is not water.

Systems for forming nanostructures are also provided. The system maycomprising a catalyst material with a surface suitable for growingnanostructures thereon, and a region in which the surface of thecatalyst material, or portion thereof, may be exposed to a set ofconditions selected to cause catalytic formation of nanostructures onthe surface of the catalyst material. In one set of embodiments, asystem includes a reaction chamber. As used herein, a “reaction chamber”refers to an apparatus within which catalytic formation ofnanostructures may take place. The reaction chamber may be constructedand arranged to be exposed to a source of a reactant vapor such that thereactant vapor may be processed to form nanostructures. In someembodiments, the reaction chamber may comprise catalyst materials asdescribed herein positioned within the reaction chamber which may beexposed to the source of the reactant vapor. As used herein, a system“constructed and arranged to be exposed to a source of a reactant vapor”is a term that would be understood by those of ordinary skill in theart, and is given its ordinary meaning in this context and, for example,refers to a system provided in a manner to direct the passage of a fluid(e.g., vapor), such as a fluid that is or that includes a hydrocarbon,over the catalyst material positioned within the reaction chamber. The“source of a reactant vapor” may include any apparatus comprising areactant vapor, any apparatus or material that may be used to produce areactant vapor, and the like. A “reactant vapor” as used herein refersto a gas or mixture of gases that may include a hydrocarbon (e.g.,ethylene, etc.) and/or other components, including hydrogen, helium,and/or other additives such as alkynes. The reaction chamber may alsoinclude an outlet via which a fluid, such as a product vapor, may exitupon completion of the reaction.

As used herein, the term “nanostructure” refers to elongated chemicalstructures having a diameter on the order of nanometers and a length onthe order of microns to millimeters, resulting in an aspect ratiogreater than 10, 100, 1000, 10,000, or greater. In some cases, thenanostructure may have a diameter less than 1 μm, less than 100 nm, 50nm, less than 25 nm, less than 10 nm, or, in some cases, less than 1 nm.Typically, the nanostructure may have a cylindrical orpseudo-cylindrical shape. In some cases, the nanostructure may be ananotube, such as a carbon nanotube.

As used herein, the term “nanotube” is given its ordinary meaning in theart and refers to a substantially cylindrical molecule or nanostructurecomprising a fused network of primarily six-membered aromatic rings. Insome cases, nanotubes may resemble a sheet of graphite formed into aseamless cylindrical structure. It should be understood that thenanotube may also comprise rings or lattice structures other thansix-membered rings. Typically, at least one end of the nanotube may becapped, i.e., with a curved or nonplanar aromatic group. Nanotubes mayhave a diameter of the order of nanometers and a length on the order ofmillimeters, or, on the order of tenths of microns, resulting in anaspect ratio greater than 100, 1000, 10,000, or greater. In some cases,the nanotube is a carbon nanotube. The term “carbon nanotube” refers tonanotubes comprising primarily carbon atoms and includes single-wallednanotubes (SWNTs), double-walled CNTs (DWNTs), multi-walled nanotubes(MWNTs) (e.g., concentric carbon nanotubes), inorganic derivativesthereof, and the like. In some embodiments, the carbon nanotube is asingle-walled carbon nanotube. In some cases, the carbon nanotube is amulti-walled carbon nanotube (e.g., a double-walled carbon nanotube). Insome cases, the nanotube may have a diameter less than 1 μm, less than100 nm, 50 nm, less than 25 nm, less than 10 nm, or, in some cases, lessthan 1 nm.

It should be understood that formation of nanotubes is described hereinby way of example only, and that other nanostructures may also be formedusing methods of the invention, including nanotubes, nanowires,nanofibers, and the like.

The catalyst material may be any material capable of catalyzing growthof nanotubes. The material may be selected to have high catalyticactivity and/or compatibility with a substrate, such that the catalystmaterial may be deposited or otherwise formed on the surface of thegrowth substrate. For example, the catalyst material may be selected tohave a suitable thermal expansion coefficient as the substrate to reduceor prevent delamination or cracks. The catalyst material may bepositioned on or in the surface of the growth substrate. In some cases,the catalyst material may be formed as a coating or pattern on thesurface of the substrate, using known methods such as lithography. Inother embodiments, the substrate may be coated or patterned with thecatalyst material by contacting at least a portion of the substrate witha solution, film, or tape comprising the catalyst material, or precursorthereof.

Materials suitable for use as the catalyst material include metals, forexample, a Group 1-17 metal, a Group 2-14 metal, a Group 8-10 metal, ora combination of one or more of these. Elements from Group 8 that may beused in the present invention may include, for example, iron, ruthenium,or osmium. Elements from Group 9 that may be used in the presentinvention may include, for example, cobalt, rhenium, or iridium.Elements from Group 10 that may be used in the present invention mayinclude, for example, nickel, palladium, or platinum. In some cases, thecatalyst material is iron, cobalt, or nickel. In an illustrativeembodiment, the catalyst material may be iron nanoparticles, orprecursors thereof, arranged in a pattern on the surface of the growthsubstrate. The catalyst material may also be other metal-containingspecies, such as metal oxides, metal nitrides, etc. For example, thecatalyst material may be a metal nanoparticle. Those of ordinary skillin the art would be able to select the appropriate catalyst material tosuit a particular application.

In some embodiments, the catalyst may comprise iron. For example, ironmay be formed on the surface of a substrate (e.g., a silicon substrate).In some embodiments, the substrate may comprise silicon. In someembodiments, the substrate may comprise aluminum oxide formed onsilicon.

In some cases, nanotubes may be synthesized using the appropriatecombination of nanotube precursors and/or catalyst materials. In someembodiments, the nanotube precursors may be delivered sequentially orsimultaneously (e.g., as a mixture of nanotube precursors).

The catalyst material may be formed on the surface of the growthsubstrate using various methods, including chemical vapor deposition,Langmuir-Blodgett techniques, deposition from a solution of catalystmaterial, or the like.

The substrate may be any material capable of supporting catalystmaterials and/or nanostructures as described herein. The growthsubstrate may be selected to be inert to and/or stable under sets ofconditions used in a particular process, such as nanostructure growthconditions, nanostructure removal conditions, and the like. In somecases, the growth substrate may comprise alumina, silicon, carbon, aceramic, or a metal. In some embodiments, the growth substrate comprisesAl₂O₃ or SiO₂ and the catalyst material comprises iron, cobalt, ornickel. In some cases, the growth substrate comprises Al₂O₃ and thecatalyst material comprises iron.

EXAMPLES

Using in situ CNT height measurements and complimentary gas analysis,thermally generated compounds that were correlated with CNT formationrate (e.g., methyl acetylene and vinyl acetylene) were identified. Todemonstrate that these alkynes were responsible for rapid CNT growth,each chemical and typical feedstock gases was delivered, withoutheating, directly to a locally heated metal catalyst substrate. Thetested alkynes accelerated CNT formation to rates comparable to orgreater than those achieved via thermal treatment of the feedstock gas.Ethene and hydrogen were still required for efficient CNT formation, buttheir input concentrations could be reduced by 20 and 40%, respectively,without sacrificing CNT growth rate. Using this new approach ofunheated, alkyne-assisted CNT growth, emissions of volatile organiccompounds and polycyclic aromatic hydrocarbons were reduced by more thanan order of magnitude compared to traditional CVD approaches.Furthermore, the chemical studies presented here shed new light on thecurrent understanding of CNT synthesis, suggesting that ametal-catalyzed polymerization reaction, rather than a metal-mediatedprecipitation reaction, is responsible for CNT formation.

To improve efficiency of CNT formation, studies were conducted toinvestigate the mechanism(s) of CNT formation mechanism(s). Typically, aCVD process involves the introduction of a gaseous carbon precursor(e.g., CO, C₂H₄, or CH₄) to a heated (e.g., 700-1000° C.) reaction zonethat contains a free-floating or substrate-supported metal catalyst(e.g., Fe, Ni, or Co). CNT growth process is generally described bythree stages: nucleation, elongation, and termination. Without wishingto be bound by theory, a proposed mechanism involves dissociation of thecarbon-containing precursor at the metal catalyst during nucleation toform a CNT cap. During elongation, carbon can add to the growing CNT bycontinual dissociation at, diffusion into, and/or precipitation from themetal catalyst (i.e., the vapor-liquid-solid (VLS) model). This additionof single carbon units may continue until termination, where mechanicalstress, catalyst encapsulation, and/or catalyst deactivation may haltCNT growth.

Recent studies have shown that, in some cases, distinct effects canarise from the independent thermal treatment of the carbonaceousfeedstock and the metal catalyst. In particular, heating and cooling(“pre-heating”) the feedstock gas prior to impingement on a metalcatalyst is often necessary for rapid growth of vertically aligned multiwalled CNTs (VA-MWCNTs), enhancing the CNT formation rate by over 2000%compared to when the gas is heated only at the catalyst. However, thisthermal pre-treatment step generates a suite of volatile organiccompounds (VOCs) (from an ethene feedstock) some of which may beresponsible for enhanced CNT formation and others of which may diminishCNT quality and present environmental and occupational concern.

As described herein, these competing processes may be substantiallyreduced and, in some cases, eliminated, by selective delivery ofcritical CNT precursors, rather than relying on thermal generation toprovide a subset of necessary reactants in a complex mixture ofchemicals. Furthermore, avoiding the thermal treatment of the feedstockgas would remove the most energetically expensive component of thesynthesis and potentially improve the carbon-to-CNT mass conversionefficiency.

In the following examples, compounds on the critical path to CNTformation were identified by monitoring in situ VA-MWCNT growth rate aspotentially important molecules were delivered directly to the catalyst.With these results, co-optimization of the synthetic process wasperformed by (1) minimizing cost by selecting potent reagent gases thatrequire minimal thermal treatment, (2) maximizing production growthrates, and (3) minimizing unwanted side-products that deteriorateproduct quality (e.g., soot and polycyclic aromatic hydrocarbons (PAHs))and that threaten the health of the public and the environment (e.g.,toxics, greenhouse gases, and compounds that promote the formation ofsecondary pollutants).

Materials.

Helium, hydrogen, ethene, methane, acetylene, and 1% acetylene (mixedwith helium) were purchased from Metro Welding (Ann Arbor, Mich.) orAirGas (all ultra high purity (UHP) grade). Before use and prior tointroduction to a mass flow controller, pure acetylene was filteredthrough a Porasil-C packed column immersed in a cyrogenic solution(acetone/N₂(l) or acetonitrile/N₂(l), both-39-41° C.) to remove acetone(which is used as a stabilizer to prevent reaction in the acetylenetank). Mass spectral analysis indicated that acetone was removed tobelow the detection limit (<ppmv) by this method. The 1% acetylenemixture did not require purification, as it was free of acetone whenshipped from the manufacturer (who employ industrial cartridge removalof acetone). Other gases, including 1,3-butadiene, but-1-ene-3-yne (1%mixture in helium), methyl acetylene, ethane, and 1-butyne, werepurchased from Air Liquide America Specialty Gas in high purity. Dilutedmixtures of methane and ethane were prepared by flushing 300-mLstainless steel (SS) tanks with the gas of interest, pressurizing thetanks to some low pressure (e.g., 5 psi), and then further pressurizingthe tank with He to approximately 250 psi. Benzene was prepared byadding over 30 mL of benzene to a 300-mL SS tank, flushing the tank withHe (without removing the benzene), and pressurizing the tank to 250 psi.In all cases where He (UHP grade) was used to prepare test gases, it wasfurther purified with a Porasil-C packed column immersed in N₂ (l).

VOC Collection and Analysis.

VOCs (and PAHs) were collected and analyzed as detailed in Plata, et al,“Early evaluation of potential environmental impacts of carbon nanotubesynthesis by chemical vapor deposition,” Environ. Sci. Technol. 2009,43, 8367-8373, the contents of which are incorporated herein byreference in its entirety for all purposes. Briefly, stainless steelcanisters were placed downstream of the reactor tube and flushed for theduration of CNT growth. Gas samples were collected just after growthtermination and sealed with stainless steel ball valves. Considering theflow rate of the gas and volume of the cylinder, these samples representa 30-second integrated signal of VOCs forming during the reaction.Simultaneous measurements of VOC composition in the effluent gases weremade using an online mass spectrometer (MS, Pfeiffer OmniStar™) andmonitoring relevant ions (m/z 2, 4, 12-18, 25-30, 32, 39-42, 44-45,51-54, 65-66, 77-78, 91). These real-time analyses indicated that gascomposition over a 30-second interval during the growth cycle wasstable. VOCs in the stainless steel canisters were quantified by gaschromatography with a flame ionization detector and thermal conductivitydetector (GD-FID-TCD with He reference gas) calibrated with standard gasmixtures. Gas samples were pre-focused using a cryogenic (N₂(l)) trap ofPorasil-C treated silica beads before injection on the HayeSep Q columnDetection limits were around 0.1 ppmv. He and H₂ were quantified usingan additional GC-TCD with a N₂ reference gas.

PAH Collection and Analysis.

Briefly, PAHs were concentrated on two consecutive, pre-cleanedpolyurethane foam (PUF) filters (3″ length×1″ diameter). These filterswere in place for the entire duration of CNT growth, and the reportedPAH abundances represent an integrated signal throughout the growthperiod. PUFs were extracted by triplicate accelerated solventextractions (ASE) with a 90:10 dichloromethane:methanol mixture at 100°C. and 1000 psi for 5 min. Each extract was concentrated by rotaryevaporation, and analyzed by GC-MS. Analyte recoveries were assessedusing surrogate standards (d₁₀-acenaphthalene, m-terphenyl, andd₁₂-perylene) and ranged from 75±1% for low molecular weight PAHs (128to 154 amu) to greater than 90±1% for higher molecular weight PAHs (>166amu). Detection limits were around 1 ng g_(C feedstock) ⁻¹ (an averageof 0.001 parts per trillion by volume).

TGA and Raman Measurements.

Results of these studies are shown in the FIG. 10. TGA measurements wereperformed using a TA Instruments Q50. Samples were oxidized in 20%oxygen and 80% helium from room temperature to 900° C. at a ramp rate of5° C. min⁻¹ with a 30 min hold. The relative abundance of CNTs andamorphous carbon was calculated using a linear least-squared fit of thedifferentiated mass loss plot.

CNT structural quality was evaluated by Raman spectroscopy (DimensionP2, Lambda Solutions, λ=533 nm), with a laser power of 20 mW and spotsize of ˜25 um. Several spectra per sample were acquired, both along themidpoint of the forest side wall and along the height of the forest sidewall. The spectra were averaged for each sample, so the reportedstandard deviation represents the variance throughout the entire forest.G/D values were calculating the area under each peak.

Example 1

The following example describes the identification of thermallygenerated CNT precursor molecules. FIG. 6B shows a schematic diagram ofan atmospheric-pressure, cold-wall CVD reactor with decoupled thermalcontrol over feedstock and catalyst. Throughout this study, feedstockgases were delivered in one of two modes: (1) with thermal treatment(T_(p)=680-1040° C., C₂H₄ and H₂ only) or (2) without thermal treatment(C₂H₄ and H₂ and a test gas, “X”). The catalyst substrate temperature(T_(s)) was 725° C., unless otherwise noted. The height of the growingCNT forest was monitored with a laser displacement sensor. Using the CVDreactor shown in FIG. 6B, the temperatures of the feedstock and catalystwere independently controlled. The C₂H₄/H₂ growth mixture was heated tovarious “pre-heat” temperatures, T_(p) (860-1040° C.), and then cooledto room temperature prior to impingement on a substrate-affixed,locally-heated metal catalyst (1 nm Fe/10 nm Al₂O₃/675 um Si).Simultaneously, the composition of gases evolved from the pre-heater byex situ gas analysis was monitored, as well as the in situ CNT growthrate by monitoring the height evolution of a vertically aligned CNT“forest” using a laser displacement sensor.

FIG. 1D shows a graph of CNT growth rate as a function of the partialpressure for some thermally generated compounds, including methane,vinyl acetylene, benzene, and methyl acetylene. Resultant concentrations(atm) from four different pre-heat temperatures (860, 920, 970, 1040°C.) are shown, and growth rate increased with temperature. The symbolsare measured gas abundances just after the pre-heater tube and the linesare the best-fit curves. The abundances of methane, benzene, and vinylacetylene were linearly related to the growth rate (all R²=0.99, n=4).The relationship between methyl acetylene and the CNT growth rate wasfit by a hyperbola (R²=0.99, n=4). In all experiments, the gas flowduring growth was C₂H₄/H₂/He=70/330/0 sccm (standard cubic centimetersper minute) and the catalyst substrate temperature was 840° C.

As pre-heat temperature increased, there were strong linear correlationsbetween the growth rate and the partial pressures of methane, benzene,and vinyl acetylene (or but-1-en-3-yne); each with a correlationcoefficient (R²) of 0.99 (n=4, FIG. 1D). As the abundance of methylacetylene (propyne) increased, the growth accelerated to a point ofapparent saturation. Thus, at high methyl acetylene concentrations,something other than precursor availability limited the rate of CNTformation. Without wishing to be bound by theory, the hyperbolicbehavior is characteristic of catalysis reactions, suggesting that themetal may be acting as a true catalyst for CNT formation, rather thansimply providing a template for highly ordered carbon precipitation.

Example 2

The following example describes a general procedure for CNT synthesis.In order to measure in situ VA-MWCNT height during the reactionprogress, a custom-built CVD reactor was used with a laser displacementsensor mounted above the growth chamber, as shown by the schematicdiagram in FIG. 6A. Traditional growth feedstock gases (C₂H₄/H₂) andannealing gases (He/H₂) are introduced via a resistively heatedpre-heater tube that was operated in two modes: (1) “on” at 1000° C. and(2) “off” at room temperature (21° C.). With the pre-heater off, testgases (e.g., 1,3-butadiene, acetylene, methyl acetylene, 1-butyne, vinylacetylene, methane, ethane, or benzene) were introduced during thegrowth phase via a secondary input line, which could be flushed to avent via a 3-way valve during the flush and anneal phases. The flow rateof the test gas was balanced with an additional helium line that wascryogenically purified with a Porasil-C column immersed in liquidnitrogen. For all experiments, the total flow of gas (C₂H₄+H₂+He+testgas) was 604 sccm, except where noted. These were introduced to acold-walled quartz reactor tube that housed a resistively heated siliconplatform, which supported the vertically aligned multi-wall carbonnanotube (VA-MWCNT) catalyst substrate. The temperature of the platformwas monitored and controlled (via feedback) by an infrared (IR)temperature sensor, and the growth rate of the VA-MWCNT forest wasmonitored using a laser displacement sensor. Effluent gases werecontinually monitored by online mass spectrometry (MS, 2 sccm samplingrate), and subsequently flushed to either a vent (during anneal andflush) or though a stainless steel sampling tank (SS Tank, to collectVOCs, He, and H₂), quartz fiber filters (to collect particles >0.2 μm),and polyurethane foams (PUFs, to collect PAHs).

In this reactor, gases were pre-mixed and introduced to a resistivelyheated quartz pre-heater tube (4×300 mm (inner diameter×length)), cooledto room temperature, and then delivered to a quartz reactor tube(4.8×22.9 cm). Inside the reactor tube, VA-MWCNT thin films were grownon electron-beam deposited Fe (1 nm, such as 1.2 nm) with an Al₂O₃ (10nm) under layer on a Si (600 μm or 675 μm) support. The catalystsubstrate temperature was regulated by a localized, resistively heatedsilicon platform, thereby minimizing gas phase reactions in the growthchamber. The temperature of the silicon platform was measured using aninfrared sensor mounted below the reactor tube and fixed at 725° C. forall of these experiments (except where noted). The temperature of the“cool-wall” reactor was less than 70° C. over the length of the platform(4-5 cm) and room temperature elsewhere, as measured by asurface-contact thermocouple place on the outside of the quartz wall.The pre-heater temperature was determined by a thermocouple placedoutside of the quartz tube in the center of the resistively heatedcoils. The pre-heater was operated in two modes: (1) “on” at 1000° C. or(2) “off” at room temperature (21° C.). In both modes, reactant vapors(C₂H₄/H₂) traveled through the pre-heater tube and connected to athree-way valve that was installed downstream of the pre-heater. Thisallowed the introduction of helium carrier gas and specific VOC testgases (e.g., methyl acetylene or vinyl acetylene) to the reactant streammixture prior to impingement on the catalyst. The sum of these flows(He+VOC test gas) was constant, but the distribution varied; test gaseswere only introduced when the pre-heater was “off.” Given the varying Hedelivery, the effects associated with trace contaminants (e.g., methaneor water) in the UHP grade He were substantially removed by purifyingthe carrier gas using a N₂ (l) cold trap with Porasil-C treated silicabeads.

While the mixture of reactant vapors varied during the growth phase, thereactor flush and annealing treatments were kept constant. A typicalreactant vapor program included: He flush at 1000 sccm or 2000 sccm for8 min (where the pre-heater is turned on after 5 min, if applicable), Heat 70 or 174 sccm, H₂ at 300 or 310 sccm, for 4 min (where the catalystsubstrate platform is turned on after 2 min), and C₂H₄ (120 sccm), H₂(310 sccm), the VOC test gas, and He were introduced for the duration ofthe CNT growth (where the summed flow rate of these gases was always 604sccm, except where noted).

Example 3

Accelerated CNT growth without heating feedstock gases. Using adecoupled CVD reactor, the temperature of VA-MWCNT metal catalyst andC₂H₄/H₂ feedstock was independently controlled while monitoring thecomposition of gases evolved from the pre-heater, as well as the in situVA-MWCNT forest growth rate. As the pre-heater temperature increased(from 690 to 1200° C.), the growth rate of the VA-MWCNT forestincreased. FIG. 1 shows various graphs illustrating the correlationbetween thermally generated compounds and increases in VA-MWCNT growthrate. In all subplots, the symbols are measured data and the line is thebest-fit curve. FIG. 1A shows methane's abundance was equally well fitby linear and logarithmic relationship with the growth rate (R²=0.99,n=4). FIG. 1B shows that benzene (circles) and vinyl acetylene (squares)were linearly related to the growth rate (R²=0.99, n=4). FIG. 1C showsthat methyl acetylene is logarithmically related to the VA-MWCNT growthrate (R²=0.99, n=4). In these experiments, the gas flow during growthwas C₂H₄/H₂/He=70/330/0 sccm and the catalyst substrate temperature was825° C. Without wishing to be bound by theory, the accelerated CNTformation may be attributed to some subset of thermally generatedcompounds, as there were strong correlations between the growth rate andthe partial pressures of methane, benzene, and vinyl acetylene (eachwith correlation coefficients of 0.99; n=4, FIG. 1). The relationshipbetween methane abundance and VA-MWCNT growth was also well fit by alogarithmic curve (R²=0.99, n=4), and methyl acetylene's relationship tothe VA-MWCNT growth was best described by a logarithmic fit (R²=0.99,n=4).

Methane is a common CVD feedstock gas and may facilitate effective CNTgrowth via decomposition on Fe catalyst surfaces. Benzene has beendiscussed as an important intermediate in CNT formation.

While alkynes (e.g., methyl acetylene and vinyl acetylene) have not beenrecognized as active molecules in the CNT formation pathway, acetylenehas been noted for its relatively efficient conversion to CNT inmolecular beam experiments (where gas phase reactions are minimized).However, in those studies, acetone was present as a trace (<1%)component of the commercially available acetylene, and it is unclear ifthe enhanced growth was an effect of the acetylene itself or theoxygen-containing acetone. (Oxygen-containing compounds can enhance CNTgrowth. Nevertheless, in homogeneous transition metal catalysis, alkyneshave been shown to react with alkenes to form cyclic compounds, whichare subsequently released from the active metal. Although CVD is aheterogeneous catalysis that may have some polymerization character, itis possible that alkynes are playing a similar role in CNT formation.

To study the effect that small alkynes (with C_(n≦4)) have onaccelerated VA-MWCNT growth as described herein, a series of potentialpre-cursor molecule were delivered to a heated metal catalyst withoutthermal treatment of the feedstock gas. To simulate the growthenvironment that would be generated by the pre-heater (without theconvolution of the more than 40 thermally generated compounds), traceamounts of the test gas (e.g., <1% by vol), along with a supply ofethene (18.7% by vol) and hydrogen (51.3% by vol, balance He), weredelivered to a heated metal catalyst.

FIG. 2 shows various graphs illustrating the effects of chemicalstructure on VA-MWCNT growth. In all experiments, standard growth gasses(C₂H₄/H₂=120/310-sccm) were delivered without pre-heating in addition toeither (1) no test gas (“pre-heater off”) or (2) a test gas (e.g.,methyl acetylene or methane). To provide a reference for typical growthconditions, a “pre-heater on” case (where only C₂H₄ and H₂ aredelivered) is also shown. In FIG. 2A, test gases were delivered at equalpartial pressures (9.8×10⁻³ atm), except for vinyl acetylene, which wasmore dilute (3.0×10⁻³ atm). All alkynes significantly accelerated growthcompared to when they were not delivered (pre-heater off case). Testgases were then delivered at lower partial pressures (3.3×10⁻⁴ atm), asshown in FIG. 2B, or equal masses (5.5±0.4 ug C sccm⁻¹), as shown inFIG. 2C.

When equal partial pressures (9.8×10⁻³ atm) of each test gas weredelivered, acetylene and methyl acetylene enhanced the growth rate ofCNTs to a greater extent than either 1,3-butadiene or methane (FIG. 2A,growth rates of 4.1 μm s⁻¹ for both acetylene and methyl acetylene, and2.9 μm s⁻¹ for pre-heater on). Notably, the mass spectral analysisshowed no acetone in the cryogenically purified acetylene and thus, therate enhancement could be attributed solely to the presence ofacetylene. The addition of ethane, an abundant component of the thermaltreatment of ethene, was not observed to accelerate CNT growth beyondthat observed without any additional test gas (the “pre-heater off”)case. A third alkyne was tested, but at a slightly lower partialpressure (3.0×10⁻³ atm), due to stability-derived concentrationlimitations on vinyl acetylene and our experimentally accessible flowrates. In spite of over a factor-of-3 dilution compared to acetylene andmethyl acetylene, the CNT growth rate of vinyl acetylene was only 1.3times slower (3.1 ™ s⁻¹ vs. 4.1 ™ s⁻¹) and still faster than thepre-heater on case (2.9 μm s⁻¹). Thus, if these concentrations arewithin the linear response range of vinyl acetylene's affect on growth(rather than some asymptotic response region as observed for methylacetylene), then vinyl acetylene could be more active than both methylacetylene and acetylene.

Additional activity of vinyl acetylene may arise from the double bond atthe head of the molecule, and to probe the effect of this functionalgroup, the accelerating effects of 1-butyne (which lacks a double bond)were studied. At the same partial pressures (3.3×10⁻⁴ atm), 1-butyneaccelerated the growth of CNTs to a lesser extent than vinyl acetylene(FIG. 2B), suggesting that the alkene group may play a role in promotingCNT formation beyond the effects of the alkyne alone, as discussed morefully below.

Vinyl acetylene, which contains both a double and a triple bond, wasobserved to accelerate the growth of CNTs to a greater extent than didethyl acetylene (or 1-butyne), which contains a triple bond, but lacksthe double bond ((0.9 vs. 0.6 μm s⁻¹, respectively; FIG. 2B), indicatingthat the alkene group may play a role in promoting CNT formation beyondthe effects of the alkyne alone. Even so, the presence of the alkynefunctional group also affects CNT formation, as its absence can, in somecases, render the molecule an inefficient promoter of CNT formation. Forexample, 1,3-butadiene, which lacks a triple bond but containsalternating double bonds, was not observed to enhance CNT growth to theextent that vinyl acetylene does (1.1 vs. 3.1 um s⁻¹, respectively; FIG.2A). It is noted that diacetylene (or 1,3-cutadiyne) was formed in casesin which accelerated growths was observed (see FIG. 3), and it maycontribute to enhanced CNT formation.

For completeness, benzene, which was correlated to VA-MWCNT growth rate,was delivered to the catalyst as a test gas. There was limitedacceleration in CNT formation at this low abundance of benzene (vaporpressure limits prevent the use of higher concentrations), but 3.3×10⁻⁴atm is within the range expected from thermal generation. Similarly,relevant concentrations of methane did not promote CNT formation rates.Thus, correlation with rate did not necessarily indicate that benzene ormethane was acting to accelerate VA-MWCNT growth at the catalyst, asthey may simply have been synthesized in sequence from another criticalcomponent (e.g., benzene formation methyl acetylene-derived radicals).

Previous studies have suggested that the perceived inactivity of methanecould be due to its low carbon content (per mol gas) as compared tolonger alkenes and alkynes. To ensure that the observed rateenhancements were not merely the result of differences in compoundmolecular mass, the delivery of each test precursor to a constant mass(5.5±0.4 ug C sccm⁻¹, where sccm is a standard cubic centimeter perminute) was normalized. In this example, on a mass basis, vinylacetylene exhibited the greatest rate acceleration, followed byacetylene, methyl acetylene, 1,3-butadiene, and methane (FIG. 2C). Thus,the accelerating effects of alkynes on CNT growth can be attributed atleast in part to the chemical structure (e.g., rather than simply torelative carbon contents).

Ultimately, rapid CNT growth can be achieved without heating feedstockgas, which will reduce energy requirements of industrial scale CVDsynthesis and likely limit the formation of unintended by-products(e.g., toxics and greenhouse gases).

Example 4

The following example describes the collection and analysis of VOCsduring the formation of carbon nanotubes, as described in Example 1.VOCs were collected and analyzed. Briefly, stainless steel canisterswere placed downstream of the reactor tube and flushed for the durationof CNT growth. Gas samples were collected just after growth terminationand sealed with stainless steel ball valves. Considering the flow rateof the gas and volume of the cylinder, these samples represented a30-second integrated signal of VOCs forming during the reaction.Simultaneous measurements of VOC composition in the effluent gases werecollected using an online mass spectrometer (MS, Pfeiffer OmniStar™) andmonitoring relevant ions (m/z 2, 4, 12-18, 25-30, 32, 39-42, 44-45,51-54, 65-66, 77-78, 91). These real-time analyses indicated that gascomposition over a 30-second interval during the growth cycle wasstable. VOCs in the stainless steel canisters were quantified by gaschromatography with a flame ionization detector and thermal conductivitydetector (GD-FID-TCD with He reference gas) calibrated with standard gasmixtures. Gas samples were pre-focused using a cryogenic (N₂ (l)) trapof Porasil-C treated silica beads before injection on the HayeSep Qcolumn Detection limits were around 0.1 ppmv. He and H₂ were quantifiedusing an additional GC-TCD with a N₂ reference gas.

Reduced by-Product Formation without Sacrificing CNT Growth.

FIGS. 3-4 show plots of the partial pressures of various reactants andbyproducts, including volatile organic compounds (VOCs), during ananostructure formation process. (“Me acetylene” and “vin acetylene” aremethyl acetylene and vinyl acetylene, respectively.) As shown in FIG. 3,eliminating thermal pre-treatment of feedstock gases reduced VOCformation. Labels shown on the abscissa indicate the identity of thetest gas and correspond to the growth curves shown in FIG. 2A. In the“pre-heater on” and “pre-heater off” case, no test gas was added to thetypical feedstock gas (C₂H₄/H₂/He=120/310/174 sccm). Error barsrepresent one standard deviation on multiple measurements and invisibleerror bars are smaller than the symbol. In FIG. 3, VOC concentrationsare reported as partial pressure. In FIG. 4, PAH abundance wasintegrated throughout the CNT growth, and so concentrations are reportedrelative to total g C delivered.

Although feedstock gases were not heated prior to impingement on themetal catalyst, they were subject to local heating in the proximity ofthe resistively heated catalyst substrate platform. Thus, gas phaserearrangements in the reactor tube could result in the formation ofunintended side products. In previous CVD approaches (e.g., with thepre-heater on), many volatile organic compounds are generated from thethermal treatment of ethene and hydrogen (FIG. 3), including methane (apotent greenhouse gas), benzene, and 1,3-butadiene (hazardous airpollutants regulated by the EPA). Omitting pre-heating can drasticallyreduce the abundance of all VOCs (except ethane) by more than an orderof magnitude, as shown in Tables 1-2 and FIG. 3. Several compounds,including benzene, 1,3-butadiyne, cyclopentadiene, pentene, and pentane,were not even formed in appreciable quantities (>0.1 ppmv) when thepre-heater was off. Thus, CNT fabrication techniques that reduce energydelivered to (e.g., that limit the thermal treatment of) the feedstockgas offer substantial reductions in unnecessary emissions. Compared topre-heating, methyl and vinyl acetylene-assisted growth reduced methaneformation by about a factor of 30 and 1,3-butadiene formation 60 fold;benzene formation was effectively eliminated (below detection; <0.1ppmv).

However, in some cases, where the pre-heating step is omitted, it may beadvantageous to utilize an alkyne additive to achieve comparable CNTgrowth rates and heights. The addition of methyl acetylene and vinylacetylene was observed to increase the VOC content of the effluent gas,but not above levels produced by thermal treatment of the feedstock.Compared to pre-heating, methane formation was reduced by about a factorof 30 during alkyne-assisted CNT growth; 1,3-butadiene formation wasreduced by more than a factor of 60; and benzene formation wasessentially eliminated. In this example, benzene was only formed when1,3-butadiene was added to the unheated feedstock gases, and VOCs wererelatively high when compared to other experiments without thermaltreatment (e.g., pre-heating. The addition of either methane or ethanedid not substantially increase the VOC load of the effluent, but it didaugment the formation of methyl acetylene in gas phase reactions aroundthe heated substrate (potentially due to the combination of methaneradicals with ethene).

TABLE 1 VOC content of test gas reaction effluents. These entriescorrespond to the CNT growth rate curves that appear in FIG. 2a. Theentries are partial pressures (atm) and within each column, theconcentration is reported before its standard deviation (shown initalics). Values should be multiplied by 10 raised to the power given inthe parentheses (e.g., 4.0(−4) = 4.0 × 10⁻⁴ atm). Pre-heater onPre-heater off +methane +ethane methane 3.7(−3) 1(−4) 6.7(−5) 6(−6)1.2(−2) 1(−3) 9.1(−5) 8(−6) ethane 1.7(−2) 1(−3) 8.8(−3) 8(−4) 7.8(−3)3(−4) 2.1(−2) 2(−3) propylene 7.1(−4) 2(−5) 2.0(−5) 2(−6) 3.7(−5) 2(−6)4.8(−5) 4(−6) propane 3.2(−5) 1(−6) 2.0(−6) 8(−7) 3.9(−6) 2(−7) 5.3(−6)4(−7) propyne 1.7(−4) 1(−5) 4.4(−7) 5(−8) 4.8(−5) 2(−6) 4.8(−5) 4(−6)1,3-butadiene 3.6(−3) 1(−4) 8.4(−5) 8(−6) 7.9(−5) 3(−6) 2.2(−4) 2(−5)but-1-en-3-yne 2.7(−4) 1(−5) 1.1(−5) 1(−6) 1.1(−5) 1(−6) 2.5(−5) 2(−6)1,2-butadiene 1.2(−4) 1(−5) 1.3(−6) 1(−7) 1.5(−6) 1(−7) 1,3-butadiyne3.0(−5) 1(−6) cyclopentadiene 1.4(−5) 1(−6) pentene 1.2(−5) 1(−6)pentane 1.7(−4) 1(−5) hexene benzene 3.1(−4) 1(−5)

TABLE 2 VOC content of test gas reaction effluents (continued). Theseentries correspond to the CNT growth rate curves that appear in FIG. 2a.The entries are partial pressures (atm) and within each column, theconcentration is reported before its standard deviation (shown initalics). Values should be multiplied by 10 raised to the power given inthe parentheses (e.g., 4.0(−4) = 4.0 × 10⁻⁴ atm). +1,3-butadiene+but-1-en-3-yne +propyne methane 4.0(−4) 1(−5) 1.0(−4) 1(−5) 3.3(−4)1(−5) ethane 9.3(−3) 1(−4) 8.5(−3) 5(−4) 6.1(−3) 1(−4) propylene 2.8(−4)1(−5) 5.8(−5) 4(−6) 1.3(−3) 1(−4) propane 1.5(−5) 2(−6) 4.2(−6) 1(−6)1.3(−4) 1(−5) propyne 1.0(−4) 1(−5) 8.9(−6) 2(−6) 8.5(−3) 1(−4)1,3-butadiene 1.0(−2) 1(−3) 5.2(−4) 3.3(−5)   1.5(−4) 1(−5)but-1-en-3-yne 6.4(−4) 1(−5) 4.3(−5) 3(−6) 2.2(−5) 1(−6) 1,2-butadiene2.8(−4) 4(−5) 1.1(−5) 1(−6) 2.2(−6) 1(−7) 1,3-butadiyne 2.4(−6) 4(−7)1.5(−6) 1(−7) cyclopentadiene 1.2(−5) 1(−6) 7.0(−6) 2(−6) pentene3.2(−6) 4(−7) 6.5(−6) 1(−7) pentane 2.5(−6) 1(−7) 3.7(−5) 1(−6) hexene3.5(−4) 1(−5) benzene 1.0(−4) 5(−5)

Example 5

The following example describes the collection and analysis of PAHsduring the formation of carbon nanotubes, as described herein. PAHs werecollected and analyzed. Briefly, PAHs were concentrated on twoconsecutive, pre-cleaned polyurethane foam (PUF) filters (3″ length×1″diameter). These filters were in place for the entire duration of CNTgrowth, and the reported PAH abundances represent an integrated signalthroughout the growth period. PUFs were extracted by triplicateaccelerated solvent extractions (ASE) with a 90:10dichloromethane:methanol mixture at 100° C. and 1000 psi for 5 min. Eachextract was concentrated by rotary evaporation, and analyzed by GC-MS.Sample recovery was assessed using internal standards(d₁₀-acenaphthalene, m-terphenyl, and d₁₂-perylene) and ranged from75±1% for low molecular weight PAHs (128 to 154 amu) to greater than90±1% for higher molecular weight PAHs (>166 amu). Detection limits werearound 1 ng g_(C feedstock) ⁻¹ (an average of 0.001 parts per trillionby volume).

Several toxic PAHs were formed and emitted during carbon nanotubeformation. Eliminating thermal treatment of the feedstock gases reducedthe total PAH load by an order of magnitude, as shown in Tables 3-4.Naphthalene, fluoranthene, and pyrene were most sensitive to changes inthermal pre-treatment, reduced by factors of 20, 60, and 40,respectively. The acenaphthylene, acenaphthene, fluorene, phenanthrene,and anthracene contents of the effluent were only slightly reduced byforegoing thermal pre-treatment. Interestingly, adding ethane resultedin a measurable reduction in fluoranthene and pyrene, and1,3-butadiene-assisted CNT growth did not yield measurable fluoroantheneor pyrene. In contrast, fluoranthene and pyrene were elevated inalkyne-assisted CNT syntheses relative to unassisted growths. Withoutwishing to be bound by theory, studies have postulated thatfluoranthene, whose structure resembles a CNT cap, is responsible forCNT nucleation. The increased abundance of these four-ringed PAHs inenhanced syntheses, and their reduced presence in reactions that showedno enhancement, may indicate their potential role in CNT formation.

The total PAH content of alkyne-assisted CNT syntheses was elevatedrelative to unassisted growths, but was reduced by over an order ofmagnitude compared to traditional thermal pre-treatment techniques.Thus, the potential environmental impact of CNT manufacture can bemarkedly reduced without sacrificing CNT growth rate. In some cases,selective delivery of important CNT precursor molecules can affordgreater control over the reaction, as thermally generated compounds thatinterfere with product quality can be minimized or even avoided. Forexample, PAHs may contribute to the formation of amorphous carbon, asignificant, interfering, and difficult to remove co-product generatedin many CNT-forming processes. The reduced PAH content ofalkyne-assisted syntheses described herein may yield high purity CNTswith limited amorphous carbon coatings.

TABLE 3 PAH content of test gas reaction effluents. These entriescorrespond to the CNT growth rate curves that appear in FIG. 2a. Theentries are mass PAH per mass C feedstock (ng g⁻¹) and within eachcolumn, the concentration is reported before its standard deviation(shown in italics). Values should be multiplied by 10 raised to thepower given in the parentheses (e.g., 8.3(3) = 8.3 × 10³ ng PAH g Cfeedstock⁻¹). Pre-heater on Pre-heater off +methane +ethane naphthalene8.3(3) 8(2) 3.6(2) 3(1)  2.8(2) 2(1)  2.4(2) 3(1)  acenaphthylene 5.4(2)1(1) 2.9(2) 1(1)  1.2(2) 1(1)  7.0(1) 3 acenaphthene 1.2(2) 1(1) 8.2(1)5 4.1(1) 1 2.2(1) 1 fluorene 5.2(1) 2 3.2(1) 1 2.7(1) 1 1.3(1) 1phenanthrene 1.1(1)  2(−1) 7.0 1 5.5 3(−1) 5.2 1(−1) anthracene 2.2 1(−1) 1.7 1(−1) 1.1 1(−1) 1.1 1(−1) fluoranthene 1.2(1)  4(−1) 2.01(−1) 1.7 1(−1)  7.4(−1) 1(−2) pyrene 2.2  1(−1)  4.5(−1) 5(−2)  4.0(−1)3(−2)  2.6(−1) 1(−2) Σ_(PAH) 9.1(3) 8(2) 7.7(2) 3(1)  4.7(2) 2(1) 3.6(2) 3(1)  gC per synthesis 2.1 2.3 2.5 2.7

TABLE 4 PAH content of test gas reaction effluents (continued). Theseentries correspond to the CNT growth rate curves that appear in FIG. 2a.The entries are mass PAH per mass C feedstock (ng g⁻¹) and within eachcolumn, the concentration is reported before its standard deviation(shown in italics). Values should be multiplied by 10 raised to thepower given in the parentheses (e.g., 8.3(3) = 8.3 × 10³ ng PAH g Cfeedstock⁻¹). +but- +1,3-butadiene 1-en-3-yne +propyne naphthalene4.6(2) 6(1)  7.2(2) 1(1)  4.7(2) 5(1)  acenaphthylene 9.0(1) 5(−1)1.6(2) 1(1)  9.4(1) 1 acenaphthene 3.1(1) 2 4.4(1) 2 3.3(1) 3 fluorene2.6(1) 1 3.8(1) 1 2.7(1) 1 phenanthrene 1.2(1) 6 1.4(1) 1 2.7(1) 1anthracene 2.4 3(−1) 2.3 1(−1) 5.8 2(−1) fluoranthene 3.4 3(−1) 3.93(−1) pyrene 1.1 2(−1) 1.7 1(−1) Σ_(PAH) 6.2(2) 6(1)  9.8(2) 1(1) 6.6(2) 5(1)  gC per synthesis 1.7 2.4 0.9

Example 6

The following example describes how increased carbon conversion can leadto high-purity CNTs. The graph in FIG. 9 illustrates how alkyne-assistedCVD can offer improved carbon conversion yields and catalystefficiencies. Carbon conversion was normalized to the mass of catalystused ((g CNT per g C precursor)×100%)/g catalyst). The catalyst wasdeposited by electron-beam evaporation at an estimated density of 7.9 ngFe mm⁻². As shown in FIG. 9, augmenting growth with trace amounts ofalkynes offered substantial improvements in carbon conversion efficiency(g CNT per g C feedstock×100%, normalized to catalyst mass). Acetylene-or methyl acetylene-assisted growth improved C conversion by factors of14 and 15 (1.2×10⁵ and 1.3×10⁵% g catalyst⁻¹), respectively, exceedingthe efficiencies that were achieved with thermal treatment approaches(8.2×10⁴% g catalyst⁻¹). In addition, dosing unheated feedstock withtrace quantities of vinyl acetylene growth offered a 7.5-foldimprovement in C conversion (6.4×10⁴% g catalyst⁻¹). In contrast,methane, ethane, and 1,3-butadiene showed relatively small enhancementin CNT formation beyond what is achieved without pre-heating.

CNT yields have also been reported in terms of “catalyst efficiency” (gCNT per g catalyst), which does not account for carbon precursor mass.In this example, without the use of oxidative etchants or thermaltreatments, catalyst efficiencies up to 1.1×10³ were observed.

Also, CNT purity (% CNT) was maintained in spite of eliminating thermaltreatment of the feedstock. FIG. 10 shows a graph illustrating howproduct purity was maintained during alkyne-assisted CVD. Purity wasdetermined by thermal gravimetric analysis (TGA; reported as % CNT) andRaman spectroscopy (reported as G/D ratio). Samples analyzed herecorrespond to those grown at the same test gas partial pressures (i.e.,FIG. 1D). In the “pre-heater on” and “pre-heater off” case, no test gaswas added to the typical feedstock gas (C₂H₄/H₂/He=120/310/174 sccm).Error bars represent the uncertainty of the TGA curve fit and onestandard deviation on triplicate Raman measurements. Large TGA errorbars resulted from the relatively small total mass generated during someexperiments, as a result of inefficient precursors (i.e., pre-heateroff, +methane, +ethane, and +1,3-butadiene) or short catalyst lifetime(i.e., +acetylene).

Vinyl acetylene-, methyl acetylene-, and acetylene-assisted growthsproduced materials with CNT purities of 86±2%, 84±3%, and 91±7%,respectively, none of which are significantly different from the CNTproduct generated by feedstock heating, as shown in FIG. 10. (87±1%;results determined by thermal gravimetric analysis; FIG. 10).

Example 7

In the following example, optimization of the reactant vapor orfeedstock is studied, including minimizing the use of ethene andhydrogen.

While a relatively small concentration of alkyne added to the unheatedethene feedstock accelerated CNT growth, the mass of alkyne was notsufficient to account for the mass of CNT formed, and additionalcompounds must have added to the growing CNT lattice. The principlefeedstock, ethylene, may serve as a co-reactant at the metal catalyst toform CNTs. If so, a reduction in the rate of CNT formation as C₂H₄abundance is reduced may be expected. As noted above, transition-metalcatalyzed cyclization reactions often rely on both an alkyne and analkene to form new carbon-carbon bonds, ultimately forming unsaturatedrings with carbon backbones. In these reactions, the metal is reduced inorder to have catalytic activity, and the electrons are necessary topromote bond formation. In CNT synthesis, most catalysts are reduced(usually with H₂) prior to CNT growth (except for, most notably,Fe(CO)₅, which is used in HiPCO® syntheses). If the catalyst continuallytransfers electrons to carbonaceous reactants to form long CNTs, H₂ (anelectron donor) may be required for sustained catalytic activity.However, hydrogen's influence may not strictly be limited to catalyticeffects, as it is also important to gas-phase reactions during CNTsynthesis. To explore the role of H₂ and C₂H₄ in an alkyne-assisted CNTsynthesis, the concentration of H₂ and C₂H₄ was varied independentlywhile fixing acetylene (the cheapest of the tested alkynes).Furthermore, studies were conducted to identify the minimal H₂ and C₂H₄quantities necessary to obtain rapid, sufficient CNT formation, so as tominimize initial feedstock costs and minimize waste.

FIG. 5 shows plots of nanostructure growth rate as a function of ethene(i.e., ethylene) partial pressure, illustrating the effects of etheneand hydrogen on nanotube growth rate and catalyst lifetime duringacetylene-assisted CNT growth. Acetone-free acetylene was delivered at1.0×10⁻³ atm in all experiments; when C₂H₄ was varied, H₂ was constant0.51 atm; when H₂ was varied, C₂H₄ was constant at 0.20 atm; helium wasused to maintain a constant total flow rate of 604 sccm. As C₂H₄abundance was reduced, a sharp decrease in CNT growth rate was observed.(FIG. 5A) This is consistent with ethene having a co-catalytic role withthe alkyne in the CNT formation reaction. In contrast, the abundance ofC₂H₄ delivered as feedstock gas did not have a clear relationship withthe catalyst lifetime. (FIG. 5B) Unless C₂H₄ concentration promotessignificant soot formation over the catalyst, there may be no a priorireason to expect that C₂H₄ levels would influence the duration ofgrowth. Thus, the initial feedstock concentration of ethene can bereduced by, for example, 20%, without sacrificing CNT growth rate oryield.

The partial pressure of H₂ (pH₂) had a significant impact on thecatalyst lifetime. (FIG. 5D) At low pH₂ (<0.31 atm), there was a sharpdecrease in the catalyst lifetime, consistent with hydrogen's role as asustained source of electrons necessary to re-reduce the catalyst afterit has been oxidized (presumably by donating electrons for metal-carbonor carbon-carbon bond formation). If H₂ were acting as a reductant, theresultant oxidized product would be H⁺ ion. The reaction would bepromoted if there were some repository for this ion, and water formedvia the initial reduction of Fe₂O₃ by H₂ or as a trace component infeedstock gases, could serve as such a receptor. Indeed, recent studieshave shown that water and other oxygen-containing molecules can prolongCNT growth, but the mechanism for this enhancement has not beenestablished. While a minimum amount of hydrogen was needed to sustaincatalyst activity, excess pH₂ reduced the catalyst lifetime. Inpolyethylene (polyethene) polymerization reactions, an abrupt increasein pH₂ can terminate chain propagation by adding to the metal catalyst,blocking monomer addition, and it is often used to control the ultimatelength of the polymer. If the CNT formation reaction has analogouspolymerization character, high pH₂ would be expected to induce atermination event, as observed. Alternatively, termination events couldbe induced by via protonation by water to yield a reductive couplingproduct (e.g., adding H to the CNT and cleaving the metal-CNT bond), andrecent observations of water-induced cleavage of CNT-catalyst contactsupport this mechanism for termination.

In addition to having a substantial influence on catalyst lifetime, lowpH₂ (<0.17 atm) affected CNT growth rate. (FIG. 5A) Without wishing tobe bound by theory, two potential explanations for this (that rely oneither gas-phase or catalyst-based reactions) include: (1) in the gasphase, inadequate hydrogen can limit the formation of VOCs (possiblydenying the catalyst of a necessary precursor) and also promotepolyacetylene formation (which may disfavor structures with long-rangeorder), and (2) at the catalyst, excess hydrocarbon-derived hydrogenmust be removed from the growing CNT backbone, and H₂-derived H radicalscould abstract these, leaving a conjugated π-system in place.

H₂ serves a multi-faceted role in the CNT formation reaction. Withoutcompromising CNT growth rate or catalyst lifetime, the amount of inputhydrogen can be reduced from 0.51 to 0.31 atm, a 40% reduction that willtranslate to cost savings for the manufacturer. As noted above, a 20%reduction in ethene input may be possible (with pH₂=0.51), and there areevident opportunities to reduce both initial feedstock costs and thetotal amount of carbonaceous material being vented to the atmosphere.Furthermore, eliminating the thermal treatment of feedstock gases bysupplying the necessary precursors directly to the metal catalystreduces the formation of potentially harmful and unintended by-products,can reduce energetic costs associated with synthesis, and can limitunnecessary damage to the environment without sacrificing productionquality.

While several embodiments of the present invention have been describedand illustrated herein, those of ordinary skill in the art will readilyenvision a variety of other means and/or structures for performing thefunctions and/or obtaining the results and/or one or more of theadvantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the present invention.More generally, those skilled in the art will readily appreciate thatall parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the teachings of thepresent invention is/are used. Those skilled in the art will recognize,or be able to ascertain using no more than routine experimentation, manyequivalents to the specific embodiments of the invention describedherein. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, the invention maybe practiced otherwise than as specifically described and claimed. Thepresent invention is directed to each individual feature, system,article, material, kit, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,kits, and/or methods, if such features, systems, articles, materials,kits, and/or methods are not mutually inconsistent, is included withinthe scope of the present invention.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Other elements may optionallybe present other than the elements specifically identified by the“and/or” clause, whether related or unrelated to those elementsspecifically identified unless clearly indicated to the contrary. Thus,as a non-limiting example, a reference to “A and/or B,” when used inconjunction with open-ended language such as “comprising” can refer, inone embodiment, to A without B (optionally including elements other thanB); in another embodiment, to B without A (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” and the like are to be understoodto be open-ended, i.e., to mean including but not limited to. Only thetransitional phrases “consisting of” and “consisting essentially of”shall be closed or semi-closed transitional phrases, respectively, asset forth in the United States Patent Office Manual of Patent ExaminingProcedures, Section 2111.03.

What is claimed: 1-57. (canceled)
 58. A method for forming carbonnanostructures, comprising: contacting a reactant vapor at a temperatureof less than 300° C. with a catalyst material to cause formation ofnanostructures, wherein the reactant vapor comprises an alkyne and analkene.
 59. A method as in claim 58, wherein the alkyne is ethyne,propyne, but-1-en-3-yne, or 1,3-butadiyne.
 60. A method as in claim 58,wherein the reactant vapor comprises ethylene, hydrogen, and an alkyne.61. A method as in claim 58, wherein the reactant vapor comprises, byvolume, 16% to 35% ethylene, 16% to 70% hydrogen, and 0.1% or lessalkyne, such that the total amount of ethylene, hydrogen, and alkyneequals 100%.
 62. A method as in claim 58, wherein the reactant vaporcomprises, by volume, 20% ethylene, 51% hydrogen, and 29% helium.
 63. Amethod as in claim 58, wherein the reactant gas is substantially free ofan oxygen-containing species, provided that the oxygen-containingspecies is not water.
 64. A method as in claim 58, wherein the act ofcontacting causes formation of a product vapor comprising at least onecarbon-containing by product.
 65. A method as in claim 58, wherein thereactant vapor is maintained at a temperature of less than 100° C. priorto contacting the catalyst material.
 66. A method as in claim 58,wherein the reactant vapor is maintained at a temperature of less than50° C. prior to contacting the catalyst material.
 67. A method as inclaim 58, wherein the reactant vapor is maintained at a temperature ofless than 25° C. prior to contacting the catalyst material.
 68. A methodas in claim 58, wherein the nanostructures are formed on a surface ofthe catalyst material.
 69. A method as in claim 58, wherein thenanostructures are single-walled carbon nanotubes or multi-walled carbonnanotubes.
 70. A method as in claim 58, wherein the nanostructures arevertically aligned multi-walled carbon nanotubes.
 71. A method as inclaim 58, wherein the catalyst material comprises iron.
 72. A method asin claim 58, wherein the act of contacting causes formation of a productvapor comprising at least one volatile organic compound (VOC) orpolycyclic aromatic hydrocarbon (PAH).
 73. A method as in claim 72,wherein the volatile organic compound is methane, ethane, propane,propene, 1,2-butadiene, 1,3-butadiene, 1,3-butadiyne, pentane, pentene,cyclopentadiene, hexene, or benzene.
 74. A method as in claim 72,wherein the polycyclic aromatic hydrocarbon is naphthalene,acenaphthalene, fluorene, phenanthrene, anthracene, fluoranthene,pyrene, chrysene, coronene, triphenylene, naphthacene, phenanthrelene,picene, fluorene, perylene, or benzopyrene.
 75. A method as in claim 73,wherein the product vapor comprises methane in an amount less than 0.4%of the total volume of product vapor that exits the reaction chamberduring formation of the nanostructure, and wherein methane issubstantially not present in the reactant vapor.
 76. A method as inclaim 73, wherein the product vapor comprises 1,3-butadiene in an amountless than 0.4% of the total volume of product vapor that exits thereaction chamber during formation of the nanostructure, and wherein1,3-butadiene is substantially not present in the reactant vapor.
 77. Amethod as in claim 74, wherein the product vapor comprises benzene in anamount less than 0.3% of the total volume of product vapor that exitsthe reaction chamber during formation of the nanostructure, and whereinbenzene is substantially not present in the reactant vapor.